(a) Field of the Invention
The present invention relates to a composite comprising a graphene oxide and a nanodiamond, a method of manufacturing the same, and a nanofluid including the same.
(b) Description of the Related Art
A nanofluid is a suspension that includes different types of nano-sized materials dispersed therein in order to overcome a limitation of thermal conductivity of a single fluid. The nanofluid includes a solvent and a filler, and water and ethylene glycol which have high thermal conductivity and high temperature stability have been generally used as the solvent. In order to increase thermal conductivity of a fluid, a nanomaterial which has excellent thermal conductivity can be added to the fluid.
Research on the nanofluid used in a cooling field of a vehicle and a cooling field of electronics has been actively performed. For example, nanoparticles have been added to engine antifreeze of the vehicle to produce an engine antifreeze having excellent heat transfer performance, and nanoparticles have been added to a transmission oil for reducing frictional force and for improving heat transfer performance. In the commercialization of a nanofluid of used in the vehicle cooling field, technology for maintaining dispersion and floating characteristics thereof may be essentially required.
Moreover, for a use in the vehicle, a stable material that does react with a material of a pipe where a coolant flows may be selected.
Currently, materials such as metal, ceramic, and carbon have attracted attention as a filler for application of the nanofluid. Although there is no limitation to a shape of the filler, a particle type filler and a fiber type filler have been mainly used for dispersion stability in a solvent. Further, precipitation can be prevented and heat can be effectively transferred in the fluid by manufacturing the filler in a nanosize.
Generally, metal and ceramic materials have high specific gravity, thus they may be precipitated when used for a long period of time. Recently, a carbon nanotube and a carbon material such as graphene having low specific gravity and high thermal conductivity have attracted attention.
Particularly, since the carbon nanotube has high thermal conductivity of about 2000 W/mK, although it may be greatly applicable as a nanofluid, it is limited to be used as a nanofluid filler because of surface hydrophobicity, low dispersion, and low suspension stability thereof.
Graphene as a carbon-based filler is formed of a monomer of carbon, and has excellent electrical, thermal, and mechanical characteristics, and particularly, its thermal conductivity is greater than about 3000 W/mK, having better thermal conductivity than the carbon nanotube. However, the grapheme has a stable two-dimensional planar structure in which hexagonal carbon structures are connected, thus it has no affinity with a solvent such as water and ethylene glycol for cooling.
Thus, in order to improve dispersion in the fluid, a graphene oxide having affinity with a polar solvent may be used. The graphene oxide may be selected as a nanofluid material due to greater dispersion than the graphene, for example, since various functional groups are applied on its surface. An epoxy (COC) group and a hydroxyl (OH) group exist in a fault plane of the graphene oxide, and a carboxyl (COOH) group and the like exist in an edge of the graphene. Those functional groups have polarity, thus they are well mixed with a generally used fluid such as water and ethylene glycol.
However, even though the graphene oxide has the high thermal conductivity, an actually used nanofluid flows through a tube used as a passage, and most of the tubes in which a fluid of a high temperature and a fluid of low temperature flow are made of a metal material. For a vehicle, for example, there are various kinds of metal materials such as aluminum, iron, steel, brass, and copper at the inside through which the antifreeze of the vehicle passes. However, most of the carbon materials may cause galvanic corrosion with these metal materials.
The galvanic corrosion occurs when two different metals are coupled and placed in a corrosion solution, and one metal is first corroded whereas the other metal is protected from the corrosion.
The galvanic corrosion may occur greater as a difference between anodic indexes as inherent potential values of the metals becomes greater. The graphite comprising carbons can be included in the highest cathode compared to a general metal. That is, when the graphite is with another metal, the graphite itself is reduced and oxidizes metals around it.
In order to overcome these drawbacks of the metal corrosion and to maintain the excellent thermal conductivity and dispersion stability of the graphene oxide, direct contact between the graphene oxide and the metal may be minimized, thermal conductivity may be maintained, and a dispersion process may be stably performed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.